NITO: From "The Beatles Recording Sessions"..."One of the sessions two tape boxes offers a tantalising question, adding to the group name The Beatles the words"+Mick Jagger?". Jagger did attend the session, ostensibly to watch the proceedings, but it is feasible that he sang backing vocals in the free-for-all choruses near the end of the song."

So maybe?

It also says that the song was the first song recorded for Yellow Sub soundtrack, but they plucked it off for the B side of the next single, so it never made the soundtrack, but WAS in the movie. John played the Clavioline piano, and sang lead and backing vocals, Paul played bass and another piano and sang backing, George played Lead Guitar and sang BV and added handclaps, while Ringo played drums and tamborine and also clapped. Eddie Kramer, 2nd engineer, reportedly contributed a little vibraphone. All started on May 11, 1967....